1. Technical Field
Aspects of the invention relate to an image processing method, a program product containing instructions to carry out the method, and an image processing apparatus employing the image processing method.
2. Related Art
Conventionally, when an image that has been captured by a scanner is to be printed, a smoothing filter is typically used to smooth the image by reducing noise components and halftone dots from the image data.
Further, in general, the image data scanned by the scanner is evenly blurred and lacks sharpness. Therefore, in order to avoid vague outlines or edges of the image scanned by a scanner, especially of characters and thin lines, edges are generally enhanced using a spatial filter that enhances edges.
It is noted that, however, when an image processing apparatus is configured such that the smoothing filter is provided and smoothing is simply applied to an image captured by the scanner, sharpness of the image is diminished, while other important information on images such as characters and lines is deteriorated. On the other hand, if an image including halftone dot areas is captured and the edge enhancement is applied by simply using the edge enhancement filter, the edge areas are enhanced while the halftone dot areas, which are desired to be rather smoothed, are also enhanced, and roughness of the image is increased.
In order to avoid the above problem, for example, the image processing apparatus configured with a circuit that enables to determine whether an image captured includes halftone dot areas is known. and if the halftone dot areas are detected, smoothing is applied to such halftone dot areas while the other edge areas are enhanced by the edge enhancement filter. If such a configuration is to be employed, however, in addition to a circuit for judgment, it becomes necessary that a process to judge the halftone areas is performed. In order to detect halftone dot areas precisely with this circuit, analysis based on peripheral data is necessary, which requires larger circuit. Even with a larger circuit, however, images captured are likely to be blurred as described above, which does not necessarily ensures a precise judgment, thus deteriorating in quality of the images.
Alternatively, the image processing apparatus may be configured such that a smoothing filter for smoothing an image is used to apply a smoothing effect only to the image judged to be halftone dot, and thereafter, the edge enhancement is applied. According to such a configuration, the inappropriate edge enhancement of the halftone dot area is avoidable, and sharpness of the edge areas is maintained. An example of such a configuration is disclosed in Japanese Patent Publication No. 8-8637.
According to the above configuration, a smoothing filter is applied to an image data, and edge signals are extracted from the image by applying an edge-extracting filter. Then, based on the extracted edge signals, edge areas are enhanced while the other areas are smoothed so that noises are removed. In this process, the same smoothing filter is used for detection of edge areas and for removing noises in order to enhance the detected edges. In order to remove halftone dot elements, when an intensive smoothing filter is applied to an area with fine characters wherein vertical lines and horizontal lines are crossed, such area may be considered as a halftone dot area and smoothed accordingly. Thus, when the image data processed by the intensive smoothing filter is output, fine characters therein may be processed similarly to the halftone dot area and may lose the textual information thereof.
Therefore, in order to separate the halftone dot areas as non-edge areas while maintaining quality of image information of fine characters, it is necessary to select the smoothing process and edge-extracting function properly. However, by use of such a filter, images of fine characters may more likely be incorrectly detected as non-edge areas in separating the image areas. Therefore, although the data processed with the smoothing filter is output as data at a non-edge area, it represents a blurred image.